Vacations ideas for Finally, the terrestrial circumstances of the observer, particularly local atmospheric conditions and the lay of the land, are a key factor determining which parts of a cometary apparition may be visible to observers. For example, most Europeans missed daytime sightings of the Great Comet of because of widespread cloudy conditions. Variability Comets are remarkably varied not only in celestial route, coma and tail size, and brightness, but also in shape and color, and in the duration of their apparitions. With respect to shape, Pliny the Elder listed ten different types of comets, ranging from the bearded to the sword and from the horn to the burning torch compare ig. In their book Comet, Carl Sagan and Ann Druyan offer what they call a bestiary of comets, with images of comets that look like a fan or horse’s mane, a fountain, a tall glass, a syringe, an angel, a fetus or rabbit, a lighthouse or ball-point pen, an arrow, and a human. A fan-shaped parabolic coma, like Tebbutt’s Comet of , may look like an angel, and an oval elliptical coma, like that of Hale-Bopp, Hyakutake, or Ikeya-Zhang, may look like a fetus in the fetal position or a baby in swaddling clothes. Comet Orbits Apollonius of Myndus, who studied with the Babylonians, the forefathers of modern comet astronomy, claims that they believed that comets were reckoned to be in the same general category as the planets, albeit with eccentric orbits which, nevertheless, could be calculated. Vacations ideas 2016.